CLOSTERIUM MONILIFERUM‐EHRENBERGII(CHAROPHYCEAE, CHLOROPHYTA) SPECIES COMPLEX VIEWED FROM THE 1506 GROUP I INTRON AND ITS2 OF NUCLEAR rDNA

To assess phylogenetic relationships and speciation modes inClosterium, we sequenced two noncoding regions of the nuclear ribosomal cistron, the 1506 group I intron in small subunit and the internal transcribed spacer 2, for a total of 58 strains of theClosterium moniliferum‐ehrenbergiispecies complex. These include both homothallic and heterothallicC. moniliferumErenberg ex Ralfs v.moniliferum, heterothallicC. moniliferumv.submoniliferum(Woronichin) Krieger, and heterothallicC. ehrenbergiiMeneghini ex Ralfs that can be divided into several mating groups. We found no or very little sequence divergence within single mating groups ofC. ehrenbergiiand among all heterothallic strains ofC. moniliferumv.moniliferumorC. moniliferumv.submoniliferum.Nevertheless, sequence divergence was much greater between those mating groups ofC. ehrenbergiiand also among the three traditional taxa.Maximum parsimony and maximum likelihood analyses showed that the taxonC. ehrenbergiiwas not monophyletic. The two varieties ofC. moniliferumappeared as a sister clade to certain mating groups ofC. ehrenbergii. Among the clades that were recovered in different trees by maximum parsimony and maximum likelihood analyses, we consistently found two large conspicuous clades: clade I consisted of mating groups A, B, C, H, K, and L ofC. ehrenbergiiwhose zygospores have smooth‐walls, and clade II contained the mating groups D, E, I, J, and S whose zygospores are scrobiculate. Phylogenetic incongruences observed are discussed from the viewpoints of the different molecular nature of the group I intron and internal transcribed spacer 2, as well as putative rapid diversification of the mating groups and probable ancient ancestral hybridization.